This application claims priority to an application entitled xe2x80x9cDevice and Method for Generating Spreading Code and Spreading Channel Signals Using Spreading Code in CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Sept. 29, 1998 and assigned Ser. No. 98-40507, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a spread spectrum device and method for a CDMA communication system, and in particular, to a device and method for generating spreading sequences.
2. Description of the Related Art
Code Division Multiple Access (CDMA) mobile communication systems have developed from an existing mobile communication standard which mainly provides voice service into the IMT-2000 standard which can provide not only voice service but also high speed data transmission service. For example, the IMT-2000 standard can provide high quality voice, moving picture, and Internet search services. In CDMA communication systems, communication links between a base station and a mobile station include a forward link for transmitting from the base station to the mobile station and a reverse link for transmitting from the mobile station to the base station.
In CDMA communication systems, the reverse link typically employs a PN (Pseudorandom Noise) code complex spreading scheme as the spread spectrum method. However, the PN code complex spreading scheme has a problem when the power amplifier has an increase in the peak-to-average power ratio (PAR) because of user data. In the reverse link, an increase in the peak-to-average ratio of transmission power causes xe2x80x98re-growth,xe2x80x99 described below, which affects the design and performance of the power amplifier in the mobile stations. The characteristic curve of the power amplifier in the mobile station has a linear area and a non-linear area. When the transmission power of the mobile station increases, the signal of the mobile station will enter the non-linear area, interfering with the frequency areas of other users, which is called the xe2x80x9cre-growthxe2x80x9d phenomenon. In order not to interfere with the frequency areas of the other users, the cell area should be reduced in size and mobile stations in a cell area should transmit to the corresponding base station at a lower transmission power. Therefore, there is a need for a spreading method which decreases PAR while minimizing the degradation of bit error rate (BER) performance which affects the overall system performance.
A description of the PN complex spreading scheme will be made herein below with reference to a transmitter in a conventional CDMA communication system.
FIG. 1 illustrates a channel transmitter, including a spread spectrum device, for a CDMA communication system. As illustrated, the channel transmitter includes an orthogonal spreader 101, a complex multiplier 102, a PN sequence generator 103 and a lowpass filtering and modulation part 104.
Referring to FIG. 1, the transmission data of each channel is applied to the orthogonal spreader 101 after channel coding, repetition and interleaving through corresponding channel coders (not shown). The orthogonal spreader 101 then multiplies the input channel data by a unique orthogonal code assigned to the corresponding channel to orthogonally spread the input channel data. Walsh codes are typically used for the orthogonal codes. The PN sequence generator 103 generates spreading sequences for spreading the transmission signals of the respective channels. PN sequences are typically used for the spreading sequences. The complex multiplier 102 complex multiplies the signals output from the orthogonal spreader 101 by the spreading sequences output from the PN sequence generator 103 to generate complex spread signals. The lowpass filtering and modulation part 104 baseband filters the complex spread signals output from the complex multiplier 102 and then converts the baseband filtered signals to RF (Radio Frequency) signals.
FIG. 2 is a detailed diagram illustrating the channel transmitter of FIG. 1 for the reverse link.
Referring to FIG. 2, the transmission data of each channel undergoes channel coding, repeating, channel interleaving and binary mapping in such a manner that a signal xe2x80x9c0xe2x80x9d is mapped to xe2x80x9c+1xe2x80x9d and a signal xe2x80x9c1xe2x80x9d to xe2x80x9cxe2x88x921xe2x80x9d, prior to being input to the corresponding channel. The data of the respective channels is multiplied by unique orthogonal codes in multipliers 111, 121, 131 and 141. In FIG. 2, channel transmitters include a pilot channel transmitter, a control channel transmitter, a supplemental channel transmitter and a fundamental channel transmitter. As stated above, Walsh codes are typically used for the orthogonal codes that spread the respective channels. The orthogonally spread data of the control channel, the supplemental channel and the fundamental channel is multiplied by gains appropriate for each channel by the first to third gain controllers 122, 132 and 142. The channel data is added by binary adders 112 and 133 and then applied to the complex multiplier 102. Herein, the outputs of the binary adders 112 and 133 will be referred to as xe2x80x9cchannelized dataxe2x80x9d.
The complex multiplier 102 multiplies the outputs of the adders 112 and 133 by spreading codes to perform spreading. As stated above, the PN codes output from the PN sequence generator 103 are used for the spreading codes. The PN codes input to the complex multiplier 102 have a rate equal to a chip rate and may have a value comprised of xe2x80x9c+1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d. Herein, unless otherwise stated, the PN codes are assumed to have a value of xe2x80x9c+1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d.
With regard to the complex multiplier 102, channelized data output from the adder 112 is applied to multipliers 113 and 143, and channelized data output from the adder 133 is applied to multipliers 123 and 134. Further, a spreading code PNi output from the PN sequence generator 103 is applied to the multipliers 113 and 123 and a spreading code PNq output from the PN sequence generator 103 is applied to the multipliers 134 and 143. In addition, outputs of the multipliers 113 and 134 are subtracted from each other by an adder 114 and then applied to a first lowpass filter 115; and outputs of the multipliers 123 and 143 are added to each other by an adder 135 and then applied to a second lowpass filter 136.
A real signal out of the outputs from the binary adder 114 is input to the first lowpass filter 115 and an imaginary signal is input to the second lowpass filter 136. Output signals of the lowpass filters 115 and 136 are gain controlled by fourth and fifth gain controllers 116 and 137, respectively, then modulated, added together, and transmitted through a transmission channel. The lowpass filtering and modulation part 104 lowpass filters and modulates the output data of the binary adders 114 and 135, and then outputs the modulated data from a binary adder 118.
Several methods have been proposed for reducing the PAR of the signals output from the first and second lowpass filters 115 and 136, and those methods are based on how the PN sequence generator 103 generates the spreading codes PNi and PNq. In general, the peak-to-average power ratio PAR depends on both zero-crossings, which occur when the signs of PNi and PNq are simultaneously changed, and hold-phase-state, which occurs when the signs of both PNi and PNq are not changed. More specifically, zero-crossings (ZC) happen when, for example, an initial state in the first quadrant transitions to the third quadrant, causing a phase shift of xcfx80. Further, a hold-phase-state, or xe2x80x9chold,xe2x80x9d happens when, for example, an initial state in the first quadrant remains in the first quadrant, causing no phase shift.
As stated above, in conventional QPSK (Quadrature Phase Shift Keying) spreading, a phase of the generated spreading codes can transition from the first quadrant to any of the second, third and fourth quadrants according to the value of the PN codes. Accordingly, when the conventional spreading code generation method is used, the PAR performance may deteriorate due to the zero-crossing phenomenon and the hold-phase-state phenomenon. Therefore, in a CDMA communication system, during spreading, the PAR is increased depending on the PNi and PNq.
It is, therefore, an object of the present invention to provide a device and method for generating a spreading sequence which can decrease the peak-to-average power ratio without degrading BER performance in a CDMA communication system.
It is another object of the present invention to provide a device and method for repeatedly generating a QPSK and xcfx80/2-DPSK (Differential Phase Shift Keying) phase-shifted PN sequence as a spreading sequence in a CDMA communication system.
It is further another object of the present invention to provide a device and method for generating a QPSK, xcfx80/2-DPSK, and zero-crossing or hold phase-shifted PN sequence as a spreading sequence in a CDMA communication system.
It is still another object of the present invention to provide a device and method for generating a spreading sequence which alternately performs a DPSK phase shift and a QPSK phase shift by mixing a PN sequence with a specific orthogonal code in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for generating a DPSK and QPSK phase-shifted spreading sequence by mixing a generated PN sequence with a previous spreading sequence, and generating a spreading sequence which alternately performs a DPSK phase shift and a QPSK phase shift by selecting a generated spreading sequence, in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for generating a spreading sequence which repeats the pattern of a QPSK phase shift, a DPSK phase shift, a zero-crossing or hold (ZCH), and a DPSK phase shift by mixing a PN sequence with a specific orthogonal code in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for generating a QPSK phase shift, a DPSK phase shift, a 180xc2x0 or 0xc2x0 phase-shift (ZCH) spreading sequence by mixing a generated PN sequence with a previous spreading sequence, and generating a spreading sequence which repeatedly performs QPSK, DPSK, zero-crossing or hold, and DPSK phase shifts by selecting the generated spreading sequence, in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for alternately generating a QPSK and xcfx80/2-DPSK phase-shifted PN sequence as a spreading sequence, and spreading/despreading a channel signal using the generated spreading sequence, in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for generating a QPSK, xcfx80/2-DPSK, zero-crossing or hold phase-shifted PN sequence as a spreading code, and spreading/despreading a channel signal using the generated spreading sequence, in a CDMA communication system.
To achieve the above and other objects, a spreading code generating device is provided for a CDMA communication system. The device is comprised of a PN sequence generator for generating PNi and PNq sequences; an orthogonal code generator for generating first and second orthogonal codes which perform DPSK state transitions at intervals of at least two chips; and a spreading code generator for generating spreading codes Ci and Cq by mixing the PNi and PNq sequences with the first and second orthogonal codes such that the present phase of the spreading codes Ci and Cq alternately generates QPSK and DPSK state transitions with respect to the phase of the previous spreading codes Ci and Cq.